Magnetic field excitation in galaxies

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Magnetic field excitation in
galaxies
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Outline

• The problem of large scale fields
• Large scale flows
• Meanfield dynamos (alpha effect due to )
• The role of cosmic rays
• MRI

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Observations
Large scale magnetic fields
Strength is compatible to the small scale
component
Axisymmetric mode dominates
Seldomly reversals
Large pitch angle
Strongly (anti) correlated with optical arms
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The problem

• A lot of processes inject small scale fields in
  the ISM
• stellar dynamos combined with winds
• shock-turbulence
• plasma instabilities
• small scale dynamos

• No problem to explain the magnetic field energy
  in galaxies
• But which processes bring coherent magnetic
  fields on a scale of several kpc?

Balsara et al. 2004
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How to do ?

• large scale flows amplifying a primordial field
• inverse turbulent cascade
• formation of stronger fields during galaxy
  mergers
• and a more rapid decay of small scale
  structures

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Large scale flow

• Rotation without diffusivity (ideal MHD) (50
  rotations)
• In a Hubble time amplification by a factor of 10
• Strong seed field is neccassary (10-7?G) on large
  scale
• Pitch angle would be less than 1o
• A lot of radial reversals
• Adding turbulent diffusion one needs a source for
  Br

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Large scale flow

• Br from nonuniform radial motions in spirals
• reversals or positive pitch angles

Otmianowska-Mazur Chiba 1995
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Inverse cascade

• inverse cascade in the turbulence
• concept of the turbulent dynamo
• still no simulations for a global galaxy possible
  
• Questions about early saturation (catastrophic
  quenching)
• turbulence in the multiphase ISM

Mean field language Alpha-effect
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Dynamo Theory
Parker 55 Steenbeck, Krause Rädler 66
div B 0
BC curl B 0 outside

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Dynamo Theory

• Differential rotation ?max 50 Gy-1
• turbulence defined by u and ?
• u 10 km s-1 and ? 0.01 Gy
• ? 1 kpc2 Gy-1 and ? 10 km s-1
• induction equation with ?-quenching

• stationary axisymmetric quadrupolar field
• field strength several ?G
• pitch angle about 20o
• maximal field strength at maximal ?

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Mean field dynamo
Dynamo numbers
Caa H h-1
CWW0 H2 h-1
tan p Ö Ca / CW
Pitch angle
Independent of h
tan p Ö Cacrit / CW 0.4 h/h0 with
h0 1 km kpc s-1 p20o
But with quenching
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Turbulence from SN
Single supernova explosions Ferrière, 1998
Pitch angle 0.1o - 1o
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Turbulence from SN
Super Bubbles Ferrière, 1998
Pitch angle 1o - 30o
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Mean flow
WW0 (1(r/rW)n)-1/n
Rotation law
(n2)
rW2
rW5
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Density wave flows
Model Brandt law of rotation, rW3 and CW41,
Ca4.7
urcu0cos(mp(f - Wpt) K r/Rmax) f(r)
ufu0sin(mp(f - Wpt) K r/Rmax) f(r)
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Density wave flows
2D stationary density wave flows disturb the
dynamo
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Density wave flows
Pitch angle is maximal at field minimum
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MHD instabilities
Supernova explosions provide enough energy
for the turbulence in the ISM
Magnetic field instabilities contribute to the
turbulence
Parker instability together with cosmic rays
MRI in galaxies
Observational hint Turbulence in low star
forming regions
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Parker instability
Hanasz et al. 2002
Parker instability
Rotation
coalesence
largescale Br
largescale B?
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Cosmic rays and Parker instability
SN-explosions with cosmic rays
Fast diffusion of cosmic rays
Parker instability
Fast expansion of pressure disturbances
More effective creation of Br
Hanasz et al. 2004
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Cosmic rays and Parker instability
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Magneto rotational instability (MRI)
Weak magnetic field
Va lt ? H
is unstable
Dziourkevitch 2004
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MRI
Dziourkevitch 2004
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MRI
Dziourkevitch 2004
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MRI
Dziourkevitch 2004
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MRI
Dziourkevitch 2004
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MRI
Dziourkevitch 2004
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Open problems
Turbulence in the ISM simulations in the box
Simulation with Superbubbles
Multicomponent gas - role of hot phase
Role of magnetic instabilities
External temporal gravitational disturbances
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Open problems
Up to which scale are magnetic fields dynamical
important
What is the influence of a more complicated mean
flow in a high Reynolds number regime
How important is the galaxy evolution for
magnetic field properties
What determines the correlation between optical
arms and magnetic arms
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Questions to Observers
Detailed observations of smaller scales 3D
field topology in SNR
Correlation to rotation curves and spiral flows
Can we see coherent structures on the large scale
during galaxy formation process
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Thank you